A medical apparatus, including a reference probe having a flexible insertion tube with a distal end for insertion into a body cavity, a pair of isolated electrodes fixedly attached to the distal end, and a position sensor fixedly located in the distal end. The apparatus also includes a supplementary probe having an electrode fixed thereto. The apparatus further includes a processor, configured to inject respective alternating currents into the pair of isolated electrodes so as to generate an electrical field therefrom, to measure a potential generated at the electrode of the supplementary probe in response to the electrical field, and to evaluate a location of the supplementary probe with respect to the reference probe in response to the measured potential and in response to a position of the distal end indicated by the position sensor.